Wastewater reclamation to generate fresh water for consumption and hygiene is of utmost importance for adequate health and sanitation. This is especially true for developing countries were access to clean water is limited. In fact, estimates from the World Health Organization reveals that approximately 1 billion people around the world, mostly in developing countries, have no access to potable water, and 2.6 billion lack access to adequate sanitation. In this sense, the world faces an increasing challenge to meet the demand for potable water. This topic is also important for long-term human missions in space where fresh water for consumption, hygiene and maintenance is necessary. The three sources for wastewater reclamation and reuse in long-term space missions are humidity condensate, hygiene wastewater and human waste, such as urine as shown in Table 1.
TABLE 1SourceMass (kg/person day)Hygiene water25.3Humidity condensate1.8Urine + flush2.0Total29.1
In general, due to the high cost of delivering supplies to space, the recovery of potable water from spacecraft wastewater is critical for life support of crewmembers in long-term missions (i.e. 120-400 days). Of the various consumables required to sustain human life in space, water accounts for the greatest percentage of material by mass. Thus, it is clear that water purification is of extreme importance for future missions to the moon, mars and abroad, but it is also imperative the recovery or recycling of unused contaminants (i.e. separated from the waste stream) into useful resources, a concept known as: in-situ resource utilization.
Water recovery system functions include wastewater collection, stabilization and storage; primary processing; secondary processing including water recovery from brines; post processing, disinfection and potable water storage. Spacecraft crews need between 3.5 and 23.4 kg of water per person for each mission day depending on mission requirements as show in Table 2 below. Conversely, spacecraft crews produce between 3.9 and 23.7 kg of wastewater depending on mission requirements. The state-of-the-art water recovery system on ISS is limited to treating only urine and condensate, which is only about 20% of the potential waste stream on long duration exploration missions which may include hygiene water, laundry water and water recovered from brines and solid wastes.
TABLE 2Kg/dayKg/dayConsumablesperson/dayWasteperson/dayGases0.8Gases1.0Oxygen0.84Carbon Dioxide1.00Water23.4Water23.7Drinking1.62Urine1.50Water content1.15Perspiration/2.28of foodRespirationFood prepa-0.79Fecal Water0.09ration WaterHygiene Water6.82Hygiene Water6.51Clothes wash12.50Clothes wash11.90Urine flush0.50Urine flush0.50Humidity0.95CondensateSolids0.6Solids0.2Food0.62Urine0.06Soaps &0.05Feces0.03personalproductsPerspiration0.02Shower & hand0.02washClothes Wash0.08TOTAL5 to 25TOTAL5 to 25
Table 2 shows values for spacecraft cabin ECLS, but are not based on any specific mission. Entries and values not shown in italics are fixed quantities determined by human metabolic requirements. Entries and values shown in italics are variable quantities dependent on mission and vehicle requirements.
FIG. 1 illustrates the regenerative Environmental Control and Life Support System (ECLSS) on ISS according to the prior art. Urine is stabilized through the addition of pretreatment chemicals (chromium trioxide and sulfuric acid) at the waste collection system. Water is recovered from the pretreated urine by the Vapor Compression Distillation (VCD) subsystem, within the Urine Processor Assembly (UPA). The UPA is currently operated at a recovery rate of 70% to avoid precipitation, which would result in hardware failure. Urine distillate, the product of the VCD subsystem, together with humidity condensate are then treated through a combination of adsorption/ion exchange processes and thermal catalysis, collectively known as the Water Processing Assembly (WPA). The multi-filtration beds of the WPA are a consumable and must be regularly replaced. While largely successful in recycling water for the ISS, this design requires significant power, stored consumables, is not certified to treat wastewater containing soaps and detergents, and requires toxic and corrosive treatment chemicals.
In 2002 the NASA Advanced Life Support Division (ALS) delineate the chief goal of the program for future manned long duration missions by stating that it is essential to: “ . . . provide life support self sufficiency for human beings to carry out research and exploration productively in space for benefits on Earth and to open the door for extended on-orbit stays and planetary exploration”. In addition, NASA-ALS highlights that the main objective to fulfill such a goal is: “ . . . to develop fully regenerative integrated system technologies that provide air, water and resource recovery from wastes”. For short term duration space missions (i.e. 15-20 days) actual NASA waste handling relies on dumping and storage, but waste treatment for missions to Mars and other near-term destinations will be more challenging due to longer mission times. Water is the most massive component aboard and it is estimated that for a long duration mission of four crew members, a total of 1918 g/CM-d of the wastes will become from humans, from which 1562 g/CM-d will be urine, based on a diet of 59 g/CM-d of solid and 1503 g/CM-d of water. Urea (60.0 g/mol) and sodium chloride (58.4 g/mol) are the most predominant wastewater contaminants found in crew urine stream with 36.2% and 21.6%, respectively. One strategy to deal with these contaminants is the reverse osmosis (RO). For instance, Lueptow et al. investigated the rejection performance of several RO membranes used presently for space missions, leading to the conclusion that urea is still hard to reject by such membranes, and accounts for more than the 39% of carbon source from the total dissolved organic carbon (DOC) in transit mission wastewater (TMW). Urea is a very small, uncharged molecule and difficult to reject by either size or charge exclusion.
Another strategy for water reclamation that has been proposed during recent years by NASA is forward osmosis (FO). Forward Osmosis (FO) is a natural process where the osmotic potential between two fluids of differing solute/solvent concentrations equalizes by the movement of solvent from the less concentrated solution to the more concentrated solution, as shown in FIG. 2. Typically this is accomplished through the use of a semi-permeable membrane that separates the two solutions and allows the solvent, but not the solute, to pass through it. The movement of water across this membrane is dictated by the water flux, Jw,Jw=A(Δπ−ΔP)  (1)where Jw is the water flux, A the water permeability constant of the membrane, Δπ is the osmotic pressure differential and ΔP is the applied pressure. In the other hand, the osmotic potential, π, of each solution is,π=iMRT  (2)where i is the dimensionless van't Hoff coefficient, M is the molarity, R is the gas constant and T is the temperature. In FO systems the wastewater, or feed, is passed on one side of the membrane and a hypertonic solution (e.g. NaCl, KCl, MgCl2 etc.), also known as osmotic agent (OA), is passed on the other. The osmotic agent (OA) can use any solute with an osmotic pressure higher than that of the feed and that does not permeate the membrane.
A continuous flow FO process can be achieved by extracting the water that transferred across the membrane into the OA. Treating the OA in a reverse osmosis, distillation, or electrodialysis system can accomplish this. The continuous form of the FO process using an RO system to extract the clean water from the OA as shown in FIG. 3, is referred to as the FO/RO configuration.
Unlike reverse osmosis (RO), which utilizes a hydraulic pressure difference, FO utilizes an osmotic pressure difference as the driving force for water diffusion across the membrane. As long as the ionic potential of water on the permeate side of the membrane is higher than that on the feed side, water will diffuse from the feed side through the semi-permeable membrane and dilute the OA. The feed stream flows are maintained at a very low hydraulic pressure and a high cross flow velocity. Therefore, potential fouling contaminants, such as solids, are not forced into the membrane pore spaces as occurs in the RO process. In 2005 Childress et al. reported on the use of a direct osmotic concentration (DOC) system as pretreatment for a reverse osmosis (RO) unit. This DOC system consisted of a direct (forward) osmosis (DO) unit followed by a second unit consisting of a DO interfaced with an osmotic distillation (OD) step before entering the RO unit. This research demonstrated efficient raw wastewater recovery of 90%, but urea was poorly rejected by the DOC, which can affect the performance of the RO unit subsequently.
Accordingly, one of the main limitations to the FO/RO process is that small polar organic molecules such as urea tend to be poorly rejected by FO and RO membranes. Contaminants such as urea tend to build up in the osmotic agent loop and eventually contaminate the product water. Therefore FO/RO has been limited to the treatment of hygiene water. In order to reject urea, Childress et al. proposed a combined direct osmotic/membrane distillation (DO/MD) and a direct osmotic/membrane osmotic distillation (DO/MOD) system. It was found that with this configurations water flux could be increased by up to 25 times and urea is completely rejected. However, although this approach is useful from a water reclamation standpoint, the use of unused components in a long-term space mission is of critical importance. Perhaps, a strategy to employ resource recovery from urea could provide a more integrated system, while avoiding the complications of membrane-based distillation.
By and large, the use of membranes in spacecraft wastewater treatment has been very limited. This is because small polar organics such as urea tend to be poorly rejected by current membranes. A system able to target urea rejection will allow the use of membranes in the treatment of urine containing wastewater. Currently, membranes are only used to treat hygiene wastewater as they perform well at rejecting surfactants. The implications of this are that one membrane based treatment system can be used that address all waste streams as opposed to having to use two different system and separate plumbing to treat hygiene and urine separately, which is problematic from a mass, power, and volume perspective. Therefore, an integrated system able to reject urea and other small polar compounds while recovering resources is to be the next-generation of water recycling systems for life support systems.
Furthermore a closer look to the other compounds found in wastewater from space mission's revealed that other small polar compounds are in large quantities. Table 3 below presents the wastewater components aboard spacecrafts where can be observed the predominance of alcohols. Therefore, other small polar compounds can be targeted to generate a cleaner environment aboard a spacecraft.
TABLE 3Wastewater componentQuantityHygiene wastewater1.2g/LHumidity condensate0.62mL/LEthanol117.98g/L2-propanol31.87g/L1,2-propanediol65.04g/LCaprolactam23.74g/L2-(2-Butoxyethoxy)3.43g/LMethanol3.85g/LFormaldehyde6.82g/LFormic acid14.02g/LPropionic acid19.95g/LZinc acetate dihydrate6.53g/LAmmonium bicarbonate39.96g/LAmmonium carbonate29.87g/LUrineUrea5g/L
Therefore, the integration of FO systems along with urea rejection or degradation technology could produce a FO/RO system that could treat urine-containing wastewater. Recent investigations have evaluated the potential of integrating forward osmosis along with biological/electrochemical technology for the treatment of wastewater and electricity generation. For example, Zhang et al. reported on the integration of a FO system into a microbial fuel cell for wastewater treatment and energy generation. This work demonstrated enhanced water flux with a power output of 4.74 W/m3′ showing it to be a feasible approach. Also, other researchers have focused on the development of osmotic membrane bioreactor for wastewater reuse, demonstrating that is a feasible and economically viable approach.
Thus, what is needed is a novel and efficient system to treat human wastewater generated inboard long duration human spacecraft.